"It's very significant, the fact that we reached this attainment three years ahead of schedule," County Executive Rich Fitzgerald said.

A release issued Tuesday says air quality improved at every monitoring site in the county in 2011, and is now in compliance with levels for fine particulate matter, which can cause asthma and other health problems.

The dirtiest air had always been found at South Allegheny High School, just downwind from the Clairton Coke Works. But for the first time ever, the air there meets federal standards.

For decades, the Clairton Coke Works has been spewing out fine particulate matter -- tiny droplets of pollution that can cause bronchitis, asthma and heart disease.

"It was bad -- couldn't even keep your windows open. It was really bad," said Bo Cross, of Glassport.

High pollution levels measured on the rooftop gauges at South Allegheny High were a major reason the county flunked federal clean air standards.

But now, thanks to major improvements at the Clairton plant, the air is getting cleaner.

People who live downwind told Channel 4 Action News reporter Paul Van Osdol that they notice a difference.

"No more smell of rotten eggs," said Donna Henderson, of Port Vue.

"You get up in the morning and your cars don't have the goop and gunk all over it as it used to be," said Alex Kelley, of Port Vue.

Even though the county meets the annual standard, it does not meet the more rigorous 24-hour standard. But a clean air watchdog group is optimistic that will happen.

"We're excited. We agree with County Executive Fitzgerald that it's great news and that we do need to stay vigilant," said Jamin Bogi, of Group Against Smog Pollution.

Jim Thompson, an Air Quality Program Manager, says the improvements are due to recent pollution controls put in at nearby and out-of-state industrial plants.

County officials say the air will only get cleaner in the next few years as the Clairton Coke Works finishes a multibillion-dollar project aimed at dramatically reducing toxic emissions.

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